1. Field of the Invention
The present invention relates to a method and apparatus for increasing integrated circuit density. In particular, the present invention relates to a method and apparatus for semiconductor package volume and height reduction with enhanced reliability and simplification of the fabrication process for stacked semiconductor dice.
2. State of the Art
Chip-On-Board (“COB”) or Board-On-Chip (“BOC”) technology is used to attach a semiconductor die to a carrier substrate such as a printed circuit board (“PCB”) and may be effected using flip-chip attachment, wire bonding, and tape automated bonding (“TAB”).
Flip-chip attachment generally consists of electrically and mechanically attaching a semiconductor die by its active surface to a carrier substrate using a pattern of discrete conductive elements therebetween. The conductive elements are generally disposed on the active surface of the die during fabrication, but may instead be disposed on the carrier substrate. The discrete conductive elements may comprise minute conductive bumps, balls or columns of various configurations. Each conductive element is placed corresponding to superimposed, mutually aligned locations of bond pads (or other I/O locations) on the semiconductor die and terminals on the carrier substrate. The semiconductor die is thus electrically and mechanically connected to the carrier substrate by, for example, reflowing conductive bumps of solder or curing conductive or conductor-filled epoxy bumps. A dielectric underfill may then be disposed between the die and the carrier substrate for environmental protection and to enhance the mechanical attachment of the die to the carrier substrate.
Wire bonding and TAB attachment techniques generally begin with attaching a semiconductor die by its back side to the surface of a carrier substrate with an appropriate adhesive, such as an epoxy or silver solder. A plurality of fine wires is discretely attached to bond pads on the semiconductor die and then extended and bonded to corresponding terminal pads on a carrier substrate. The bond wires are generally attached through one of three conventional wire bonding techniques: ultrasonic bonding, using a combination of pressure and ultrasonic vibration bursts to form a metallurgical cold weld; thermocompression bonding, using a combination of pressure and elevated temperature to form a weld; and thermosonic bonding, using a combination of pressure, elevated temperature, and ultrasonic vibration bursts. A dielectric encapsulant such as a silicone or epoxy may then be applied to protect the fine wires and bond sites. In TAB, ends of metal leads carried on an insulating tape such as a polyimide are attached to the bond pads on the semiconductor die and corresponding terminal pads on the carrier substrate.
Higher performance, lower cost, increased miniaturization of components, and greater packaging density of integrated circuits are ongoing goals of the computer industry. As new generations of integrated circuit products are released, the number of components used to fabricate them tends to decrease due to advances in technology even though the functionality of the products increases. For example, on the average, there is approximately a 10 percent decrease in components for every product generation over the previous generation having equivalent functionality.
In integrated circuit packaging, in addition to component reduction, surface mount technology has demonstrated an increase in semiconductor chip density on a single substrate or board, resulting from more compact designs and form factors and a significant increase in integrated circuit density. However, greater integrated circuit density is becoming significantly limited by the space or “real estate” available for the mounting of semiconductor dice on a carrier substrate.
One method of increasing integrated circuit density is to stack dice vertically. U.S. Pat. No. 5,012,323 (“the '323 patent”), issued Apr. 30, 1991 to Farnworth, teaches combining a pair of dice mounted on opposing sides of a lead frame. An upper, smaller die is back-bonded to the upper surfaces of the leads of the lead frame via a first adhesively coated, insulated film layer. A lower, larger die is face-bonded to the lower surface of the lead frame at a die-bonding region via a second, adhesively coated, insulative, film layer. The wire-bonding pads on both the upper die and lower die are interconnected with the ends of their associated lead extensions with gold or aluminum bond wires. The lower die must be slightly larger than the upper die in order for the die pads to be accessible from above through a bonding window in the lead frame such that wire connections can be made to the lead extensions. This arrangement has a major disadvantage from a production standpoint, due to the requirement for different-sized dice, a somewhat complex lead frame configuration and a somewhat difficult-to-execute wire bonding protocol.
U.S. Pat. No. 5,291,061, issued Mar. 1, 1994 to Ball, teaches a multiple stacked die device containing up to four stacked dice supported on a die-attach paddle of a lead frame, the assembly purportedly not exceeding the height of then-current single die packages, and wherein the bond pads of each die are wire bonded to lead fingers. The relatively low profile of the device is achieved by close-tolerance stacking which is made possible by a low-loop-profile wire bonding operation and disposition of adhesive layers between the stacked dice. However, Ball requires relatively long bond wires to electrically connect the stacked dice to the lead frame. These long bond wires increase resistance and may facilitate shorting due to bond wire sweep during encapsulation. Also, Ball requires the use of spacers between the dice.
U.S. Pat. No. 5,323,060, issued Jun. 21, 1994 to Fogal et al., teaches a multichip module that contains stacked die devices, the terminals or bond pads of which are wire bonded to a substrate or to adjacent die devices. However, as discussed with Ball, Fogal requires long bond wires to electrically connect the stacked die bond pads to the substrate. Fogal also requires the use of spacers between the dice.
U.S. Pat. Nos. 5,422,435 and 5,495,398 to Takiar et al. (“Takiar”) teach stacked dice having bond wires extending to each other and to the leads of a carrier member such as a lead frame. However, Takiar also has the problem of long bond wires, as well as requiring specific sized or custom designed dice to achieve a properly stacked combination.
U.S. Pat. No. 5,973,403, issued Oct. 26, 1999 to Wark, teaches stacked dice on a PCB. A first die is flip-chip mounted on the PCB, a second die is back side bonded on the first die, and bond wires extend from the second die to the PCB. However, Wark also has the problem of long bond wires and the excessive and somewhat complex processing steps of both underfilling the first flip-chip mounted die and encapsulating the second die connected with bond wires.
U.S. Pat. No. 6,051,878, issued Apr. 18, 2000 to Akram et al. (“Akram”), teaches multiple stacked substrates having dice mounted thereto using both wire bonding and flip-chip techniques, where some dice are mounted on opposing substrate surfaces.
Therefore, it would be advantageous to develop a fairly straightforward technique and resulting semiconductor device assembly for increasing integrated circuit density of vertically stacked semiconductor dice by reducing the number of process steps for fabricating the assembly in combination while reducing assembly height and employing commercially available, widely practiced semiconductor device fabrication techniques.